Microchannel devices are finding increased use in the separation, identification and synthesis of a wide range of chemical and biological species. Employing transverse channel dimensions in the range from a few microns to about one millimeter, such systems may permit the miniaturization and large-scale integration of many chemical processes in a manner analogous to that already achieved in microelectronics. Applications for microchannel devices now under development include such diverse processes as DNA sequencing, immunochromatography, the identification of explosives, identification of chemical and biological warfare agents, and the synthesis of chemicals and drugs.
In addition to the potential for large-scale integration, the small physical scales of microchannel devices offer a few inherent advantages over their traditional macro-scale counterparts. Traditional chromatographic separations are usually performed in packed columns. The role of the packing is to provide a stationary phase having a large specific surface area for the adsorption of chemical species. Since various chemical species have different absorption probabilities and residence times on the stationary phase, they move with different speeds through the column and thus exhibit a range of arrival times at the column exit. Although larger surface areas provide better separation between arrival times, the use of packing materials causes nonuniformity of the fluid motion. This results in hydrodynamic dispersion of the solute bands or peaks used to distinguish the species. The benefit of the packing material is thus partially offset by the detriment of increased dispersion. This compromise can be avoided at the smaller scale of microchannel devices. Since the specific surface area of a tube or channel is inversely proportional to its minimum transverse dimension, microchannel columns may provide the required surface area without any need for a packing.
One promising method of microchannel separation is electrochromatography in which electric fields are used to drive electroosmotic fluid motion. Such fluid motion results from the applied electric field acting on charges in the electric Debye layer adjacent to the tube or channel walls, inducing a shear stress very near the boundary of the interior fluid. Electroosmotic flows offer two important benefits over pressure-driven flows for the small physical dimensions characteristic of microchannel devices. First, fluid speeds in electroosmotic flows are independent of the transverse tube or channel dimension over a wide range of conditions, making this technique extensible to extremely small physical scales. In contrast, pressure-driven flows require a pressure gradient that increases inversely with the square of the minimum transverse dimension to maintain a given fluid speed.
The second, more important advantage of electroosmotic flows is that the profile of the fluid velocity across a tube or channel is essentially flat over a very broad range of conditions. All variations in the axial velocity are confined to a small region adjacent to the tube or channel walls. The thickness of this fluid-dynamic boundary layer is comparable to the thickness of the electric Debye layer. The benefit of this flat velocity profile is that samples may be transported over long ranges with very little hydrodynamic dispersion due to nonuniform fluid speeds. Although dispersion in both electroosmotic and pressure-driven flows grows as the square of the Peclet number, the coefficient of dispersion in an electroosmotic flow may be many orders-of-magnitude smaller than that for the parabolic velocity profile of a pressure-driven flow. In addition, this low coefficient of dispersion permits optimum operation of an electroosmotic flow at very high Peclet numbers. This minimizes the role of ordinary diffusion in electroosmotic flows, thus offering the potential for long-range transport in chromatographic columns with little axial spreading of solute peaks due to either dispersion or diffusion. This is critical to chromatographic processes that identify chemical species by distinguishing the arrival times of closely spaced peaks or bands.
Another promising approach to microscale chemical analysis is electrophoretic separation. Here the carrier fluid may be either moving or nearly stationary, and an applied electric field is used to drive ionic species through a gel or liquid. Separation occurs because the ion speeds depend on the unique charge and mobility of each species. Provided that the applied field is uniform across the tube or channel cross-section, all ions of the same charge and mobility move at the same speed and so progress along the column without any induced dispersion. Such motion is analogous to the flat velocity profile of an electroosmotic flow, and the various species thus again exhibit unique arrival times at the column exit. Like electrochromatographic processes, electrophoretic separations may be severely degraded by diffusion or dispersion. In the latter case, however, dispersion may arise not only from nonuniformity of the carrier fluid speed but may also arise directly from nonuniformity of the electric field across the column cross-section.
Although species motion in both electrophoretic and electroosmotic transport may be relatively free of both diffusive and dispersive spreading in straight tubes or channels having parallel walls, any local variation in the fluid speed or local field strength introduces dramatic skewing of an otherwise flat interface or species band. Such skewing is known to occur in turns because the fluid moving along the outer radius of a turn must travel further than that moving along the inner radius. This difference in path length is compounded by the electric field gradient which is greater along the shorter inner radius, resulting in a greater fluid speed along the shorter path. Thus, an initially flat interface will be severely skewed in passing through a turn. Moreover, because transverse diffusion quickly redistributes solute concentrations across the channel, such skewing is irreversible, and the net effect of transport through any turn or junction is a large and permanent broadening of any solute peak or interface. Although there have been a number of published studies which demonstrate and quantify the dispersive effects of turns .sup.3, 4, 5, 6 there have been few prior attempts to remedy the problem. As a result, separations are generally performed in straight channels that are limited in length by the maximum substrate dimension. This restriction limits the separation between peaks traveling at different speeds and, so, limits the resolution of separation devices.
Beyond separation processes, sample dispersion also plays an important role in the routine transport of a sample through portions of a microchannel system. In many instances, samples are injected into the channel system via a reservoir and then transported to some remote location where the intended process is performed. Sample dispersion in turns and junctions en route to the new location results in smearing of the sample and the eventual arrival of the sample in a progressive manner spanning some extended period of time. For processes such as mixing, dilution and reaction requiring simultaneous arrival of more than one species or arrival of a sample at some prescribed concentration, such dispersion will thus impair the system performance. Likewise, synthesis via chemical reaction may require precise concentrations and the concurrent transport of multiple species side-by-side in a single channel. In such a case, the dispersion induced by a turn or junction may physically separate the reactants by moving one species ahead of or behind the other. Minimizing dispersion in microchannel devices is thus important to system performance even for processes other than species separation and identification.
Two different approaches have been used in previous efforts to minimize the dispersion induced by turns.sup.1 and by contractions at the ends of separation channels..sup.2 Nordman.sup.2 utilizes focusing electrodes to obtain a more uniform electric field and, hence, a more uniform flow field in the transition between a separation capillary and a detection region. Similar electrodes might also be used to alter the flow field and reduce dispersion in other types of junctions but only at the cost of greatly increased complexity in fabrication and control of required electrodes and circuitry, particularly in large systems having a multitude of turns and junctions. The more passive approach of Kopf-Sill and Parce.sup.1 seeks to reduce dispersion in turns by the use of advantageous channel geometries. In particular, they recommend channels having small aspect ratios such that the channel depths are much greater than their widths. The smaller channel width helps to reduce the difference in transit time along the inner and outer walls of a turn, thereby reducing dispersion. Kopf-Sill and Parce also suggest that dispersion can be reduced by fabrication of turns having a depth along the inner radius that is greater than that along the outer radius, thereby reducing the fluid speed along the inner radius. Although these measures are capable of reducing dispersion, channel aspect ratios near or above unity have a number of advantages, particularly in fabrication and in the introduction of a packing material or pattern of obstacles needed to increase surface area or to increase species selectivity. Moreover, fabrication of the variable depth channels suggested by Kopf-Sill and Parce would substantially increase costs, since most conventional lithographic and etching processes produce channels, channel molds, or embossing tools having a uniform feature depth.
Kopf-Sill and Parce also indicate that dispersion can be reduced by narrowing the channel around a turn, but go on to state that this increases resistance through the channel, lowers throughput, and causes increased current heating and higher pressures. As a result of these perceived problems, they do not pursue this approach but instead describe only deep narrow channels having an aspect ratio less than one.